A basic optical soliton pulse transmission system is similar to a "linear" transmission system, with the principal differences that the soliton system uses Return to Zero pulses of sech.sup.2 shape (instead of NRZ pulses), and transmits them along fibre with anomalous dispersion (instead of low normal dispersion). Soliton transmission systems, though offering the potential of higher capacity than `linear` optically amplified systems, are limited by several additional effects, particularly interaction between adjacent soliton pulses, noise induced pulse jitter (Gordon-Haus) effect, and constraints on the power of the solitons leading to noise problems.
These problems can be ameliorated by reshaping the pulses in their passage along the transmission path without going to the extent of a full regeneration. Thus M. Nakazawa et al have described, in a paper entitled `10 Gbit/s Soliton Data Transmission over One Million Kilometers`, Electronics Letters, Jul. 14th 1991, Vol. 27, No. 14, pp 1270-2, how soliton pulses can, at intervals along the route, be reshaped by passage through sinusoidal modulators. An alternative approach, which similarly involves introducing additional components along the route, involves passage of the soliton pulses through frequency selective filters. The Gordon-Haus effect produces unwanted frequency shifts in the solitons, and these frequency shifts give rise to jitter as the result of the effects of dispersion. Successive filters at intervals along the route selectively attenuate the spectrum and minimise the extent to which the soliton centre frequency can drift away from the frequency with which it was initially launched into the system. Such filters thus operate to constrain the magnitude of the Gordon-Haus jitter. One advantage of this approach is that the filters may be interference type filters having a regular array of pass and stop bands that can be arranged to have a spectral periodicity suitable for use in a wavelength division multiplexed (WDM) soliton pulse transmission system. On the other hand if, as originally proposed, all the filters are tuned to the same frequency, then accumulated amplifier noise, amplified spontaneous emission (ASE), is able to propagate along the route in a manner that is inhibited by the previously referred to modulator approach. This noise is worse than in the absence of the filters because extra amplification has had to be included in the system in order to offset the loss penalty of the filters. This problem is addressed by L. F. Mollenauer et al, in a paper entitled `The Sliding-frequency guiding filter: an improved form of soliton jitter control`, Optics Letters, Nov. 15, 1992, Vol. 17, No. 22, pp. 1575-7. In this paper it is proposed that there is a small consistently positive, or consistently negative, frequency offset between the centre frequency of each filter in the system and that of the filter immediately preceding it. The paper specifically considers a numerical simulation of a system in which the aggregate centre frequency offset over a distance of 10,000 km is 56 GHz and amounts to three times the bandwidth of an individual soliton. The frequency offset is spread over approximately 350 concatenated filters, one at each amplifier. The soliton pulses are able to get through the system because a soliton pulse is able gradually to readjust its spectrum as it proceeds. This contrasts with the situation in respect of the (linear) noise, most of which is blocked because the filter pass bands down the whole system do not overlap. Thus it is only the noise generated in the last few amplifiers that is able to reach the far end of the system. The small offset in centre frequency of each filter biases, in the direction of the offset, the transmitted spectrum of every soliton pulse that passes through it. Some power is thereby lost but, provided that there is sufficient optical amplification to compensate, the soliton adjusts to become a soliton at a new offset frequency. Provided that this is done repeatedly in a large number of very small steps, then the soliton centre frequency can be made to shift gradually as it progresses down the system.
The concluding paragraph of this paper asserts certain advantages to flow from the fact that this system of jitter control uses entirely passive filters. This is specifically contrasted in this paper with the time-domain filtering of the previously referenced M. Nakazawa et al. paper, which uses active devices that this (L. F. Mollenauer et al.) paper characterises as having `all the same drawbacks of complexity reduced reliability high cost and incompatibility with wavelength division multiplexing that accompany electronic regeneration`.
Against this alleged advantage pertaining to the use of passive filters, it must be recognised that the practical realisation of approximately 350 filters, each with a centre frequency shifted by approximately 157 MHz with respect to the centre frequencies of its immediate neighbours, is an onerous task not mitigated by the fact that, if the system is to be suitable for submarine transmission system applications, it is typically going to be necessary for stability to be maintained over 25 years.